This invention relates generally to liquid ink development of images in an electrostatographic printing machine, and more particularly, concerns a method for improving transfer of develop images onto receiving substrates.
Generally, the process of electrostatographic copying is initiated by exposing a light image of an original document onto a substantially uniformly charged photoreceptive member, resulting in the creation of a latent electrostatic image of the original document on the photoreceptive member. This latent image is subsequently developed into a visible image by a process in which developer material is deposited onto the surface of the photoreceptive member. Typically, this developer material comprises carrier granules having toner particles adhering triboelectrically thereto, wherein the toner particles are electrostatically attracted from the carrier granules to the latent image for forming a developed powder image on the photoreceptive member.
Alternatively, liquid developing materials comprising a liquid carrier having toner particles immersed therein have been successfully utilized to develop electrostatic latent images, wherein the liquid developing material is applied to the photoconductive surface with the toner particles being attracted toward the image areas of the latent image to form a developed liquid image on the photoreceptive member. Regardless of the type of developing material employed, the toner particles of the developed image are subsequently transferred from the photoreceptive member to a copy substrate, either directly or by way of an intermediate transfer member. Thereafter, the image may be permanently affixed to the copy substrate for providing a xe2x80x9chard copyxe2x80x9d reproduction or print of the original document or file. In a final step, the photoreceptive member is cleaned to remove any charge and/or residual developing material from the photoconductive surface in preparation for subsequent imaging cycles.
The above described electrostatographic reproduction process is well known and is useful for light lens copying from an original as well as for printing applications involving electronically generated or stored originals. Analogous processes also exist in other printing applications such as, for example, digital laser printing where a latent image is formed on the photoconductive surface via a modulated laser beam, or ionographic printing and reproduction where charge is deposited on a charge retentive surface in response to electronically generated or stored images. Some of these printing processes develop toner on the discharged area, known as DAD, or xe2x80x9cwrite blackxe2x80x9d systems, as distinguished from so-called light lens generated image systems which develop toner on the charged areas, also known as CAD, or xe2x80x9cwrite whitexe2x80x9d systems. The subject invention applies to both such systems.
The use of liquid developer materials in imaging processes is well known. Likewise, the art of developing electrostatographic latent images formed on a photoconductive surface with liquid developer materials is also well known. Indeed, various types of liquid developing materials and liquid based development systems have heretofore been disclosed with respect to electrostatographic printing machines.
Liquid developers have many advantages, and often produce images of higher quality than images formed with dry developing materials. For example, the toner particles utilized in liquid developing materials can be made to be very small without the resultant problems typically associated with small particle powder toners, such as airborne contamination which can adversely affect machine reliability and can create potential health hazards. The use of very small toner particles is particularly advantageous in multicolor processes wherein multiple layers of toner generate the final multicolor output image.
Further, full color prints made with liquid developers can be processed to a substantially uniform finish, whereas uniformity of finish is difficult to achieve with powder toners due to variations in the toner pile height as well as a need for thermal fusion, among other factors. Full color imaging with liquid developers is also economically attractive, particularly if surplus liquid carrier containing the toner particles can be economically recovered without cross contamination of colorants.
Liquid developer material typically contains about 2 percent by weight of fine solid particulate toner material dispersed in the liquid carrier, typically a hydrocarbon. After development of the latent image, the developed image on the photoreceptor may contain about 12 percent by weight of the particulate toner in the liquid hydrocarbon carrier. However, at this percent by weight of toner particles, developed liquid images tend to exhibit poor cohesive behavior which results in image smear during transfer. In addition, partial image removal, or so-called scavenging, is problematic during successive liquid development steps, particularly in image-on-image color processes. In order to prevent image scavenging and to improve the quality of transfer of the developed image to a copy sheet, the liquid developing material making up the developed liquid image may be xe2x80x9cconditionedxe2x80x9d by compressing or compacting the toner particles in the developed image and removing carrier liquid therefrom for increasing the toner solids content thereof. The liquid toner image may be conditioned on the surface of a development member, before developing the electrostatic image on the photoreceptor; on the surface of the photoreceptor, after development; and/or on the surface of an intermediate transfer member. We will generally provide examples of conditioning on the photoreceptor surface, but it should be understood that these methods may also be used to condition the image on other surfaces. Liquid ink conditioning greatly improves the ability of the toner particles to form a high resolution image on the final support substrate or an intermediate transfer member, if one is employed.
Various devices and systems are known for effectively conditioning liquid developing materials in electrostatographic systems. In one exemplary system particularly relevant to the present invention, a device and method for increasing the solid content of an image formed from a liquid developer is provided, wherein an absorptive blotting material is contacted with the developed liquid image. A vacuum source is coupled to the blotting material so that absorbed liquid dispersant is drawn through the blotting material. The absorptive blotting material is preferably provided in the form of a covering on a porous conductive roller which is biased with an electrical charge having a polarity which is the same as the charge of the toner particles in the developing material, such that the resulting electric field repels the toner particles from the absorptive blotting material for transferring so that minimal toner particles adhere thereto.
Although various systems have been developed for conditioning an image in liquid based electrostatographic printing systems, some problems and inadequacies remain with respect to known electrostatically based systems. In particular, notwithstanding blotting rolls add additional equipment, thus increasing the cost and complexity of the marking engine. This additional cost and complexity is even greater if vacuum is used. And the blotting may disturb the image: some toner particles may transfer to the blotting roll; the high pressure from the roll and from the roll""s electrical bias may push some toner laterally on the surface of the photoreceptor.
The present invention is directed toward liquid electrophotographic reproduction machine including an image bearing member movable along a process path; latent image means mounted along the process path for forming a latent image electrostatically on said image bearing member; a development unit mounted along the process path and containing liquid developer material including a liquid carrier and dispersed charged toner particles for developing the latent image to form a toner image; an intermediate transfer member and a transfer station for transferring the toner image from the photoreceptor to the intermediate transfer member; a pre transfer station for increasing the cohesiveness of the toner image on the intermediate transfer member; a transfix station for transferring and fixing the toner image to the final copy substrate (e.g., paper, transparency, canvas, cloth); and a cleaning station for removing untransferred toner from the photoreceptor. The pre-transfer station may include an applicator member that applies a cohesion increasing (Cl) solution on the toner image and it may include a corona producing member that increases the cohesion of the toner layer.